1. Field of the Invention
The present invention relates to an image processing apparatus and its method for converting a gradation with keeping a high frequency component of an image such as an X-ray image, and a computer-readable storage medium used therefor.
2. Related Background Art
For example, an X-ray chest image is composed of an image of a lung field through which an X-ray is easy to pass and an image of a mediastinum portion through which an X-ray is very hard to pass, and therefore pixel values exist in an extremely wide range. Accordingly it has been considered to be hard to obtain an X-ray chest image on which both of the lung field and the mediastinum portion can be observed simultaneously.
Therefore, various methods described below have been conventionally suggested as methods of resolving this problem.
First, there is a method disclosed in SPIE Vol. 626. Medicine XIV/PACSIV (1986). This method is expressed by the following formula (1):SD=A[SORG−SUS+B(SUS)]+C  (1)where SD is a pixel value after processing, SORG is an original pixel value (input pixel value), SUS is a pixel value of a low frequency image of an original image (input image), and constants A, B, and C (for example, A=3, B=0.7).
In this method, it is possible to change weights of a high frequency component (first term) and a low frequency component (second term). For example, if A=3 and B=0.7, the high frequency component is highlighted and the entire dynamic range is compressed advantageously. This method is appreciated by five radiotherapists in that the processed image is useful for a diagnosis in comparison with a non-processed image.
In addition, there is disclosed a method in Japanese Pat. No. 2509503 which is expressed by the following formula (2):SD=SORG+F[G(Px,Py)]  (2)where SD is a pixel value after processing, SORG is an original pixel value (input pixel value), and Py is an average profile in a Y direction profile and Px is an average profile in a X direction profile of an original image (input image).
Characteristics of the function f(x) is described below. First, f(0) becomes “0” in “x>Dth” and f(x) monotonously decreases with “E” as an intercept and “E/Dth” as a slope in “0≦x≦Dth” as expressed by the following formula (3):F[x]=E−(E/th)X  (3)Py=(ΣPyi)/n  (4)Px=(ΣPxi)/n  (5)where (i=1−n), Pyi, and Pxi are profiles. They are expressed by the following formula (6), for example:G=(Px,Py)=max(px,py)  (6)In this method, a density range of pixel values Dth and lower of a low frequency image is compressed.
In addition, as a similar method to the above patent gazette, there is a method referred to as a self-compensatory digital filter in “Self-compensatory Digital Filter,” National Cancer Center, Anan et al in Japan Radiation Technical Society Journal Vol. 45, Issue 8, August 1989, pp. 1,030. This method is expressed by the following formulas (7) and (8):SD=SORG+f(SUS)  (7)SUS=ΣSORG/M2  (8)where SD is a pixel value after compensation (after processing), SORG is an original pixel value (input pixel value), SUS is an average pixel value of a moving average with a mask size of M×M pixels of an original image (input image), and a monotonously decreasing function f(X) shown in FIG. 35.
Next, characteristics of the function F(SUS) is described below. First, regarding the characteristics shown in FIG. 35, f(SUS) becomes “0” if “SUS>BASE” and f(SUS) monotonously decreases with “threshold value BASE” as an intercept and SLOPE as a slope if “0≦SUS≦BASE.” Therefore when executing the above formula (7) with the original pixel value SORG as a density equivalent amount, an effect to an image is obtained such as an increase of a density in a range of a lower average density of the image.
This method is different from the method expressed by the formula (2) in a low frequency image preparing method; a low frequency image is prepared with two-dimensional data in this method while it is prepared with one-dimensional data in the formula (2). In this method, however, a density range of pixel values Dth and lower of the low frequency image is compressed in the same manner.
Furthermore, there is disclosed a method in Japanese Pat. No. 2663189 which is expressed by the following formulas (9) and (10):SD=SORG+f1(SUS)  (9)SUS=ΣSORG/M2  (10)where f1(X) is a monotonously increasing function.
Characteristics of the function f1(x) is described below. First, f1(x) becomes “0” in “x<Dth” and f1(x) monotonously decreases with “E” as an intercept and “E/Dth” as a slope in “Dth≦x” as expressed by the following formula (11):f1[x]=E−(E/th)X  (11)
Still further, there is disclosed a vivifying method of highlighting a high frequency component of an image having fixed or greater density values in Japanese Pat. No. 1530832. In this method, an extremely low frequency component is highlighted and a high frequency component having a high occupancy rate of noises is relatively reduced in order to obtain an image easy to see and to improve a diagnostic performance by preventing a false image or an increase of noises.
There is a method expressed by the following formulas (12) and (13):SD=SORG+B(SORG−SUS)  (12)SUS=ΣSORG/M2  (13)where the constant B is a variable monotonously increasing according to an increase of the SOR or SUS value. When executing the above formula (12), a high frequency component of an image can be highlighted advantageously.
The above method in SPIE Vol. 626. Medicine XIV/PACSIV (1986), however, does not have a concept of compressing a dynamic range of a fixed density range, and therefore a dynamic range of the entire image is equally compressed. Accordingly only a fixed density range cannot be compressed, by which if this method is used for a lung front image, for example, there is a problem that not only the mediastinum portion but a density range of a lung portion useful for a diagnosis is compressed and therefore the diagnostic function is degraded in comparison with a method of compressing only the mediastinum portion.
The above “self-compensatory digital filter” method has a problem that an unnatural distortion may be generated in a high frequency component unless a form of the above function f(SUS) is adapted to decrease to BASE at a fixed ratio (it must be linear). Therefore, it has a problem that a gradation cannot be compressed non-linearly and freely with keeping an amplitude of a high frequency component at an amplitude of a high frequency component of the original image (input image).
In addition, generally an image whose dynamic range has been compressed is converted in its gradation again in a CRT display or a film output. The above “self-compensatory digital filter” method or the like does not have a concept of adjusting an amplitude of a high frequency component of the image after the gradation is converted, and therefore the image whose dynamic range has been compressed is further non-linearly converted in its gradation in a film output and an image display. Accordingly, the conversion depends upon a slope of a gradation conversion curve and the amplitude of the high frequency component fluctuates. Therefore, it has problems that the amplitude after the gradation conversion is distorted non-linearly and that the amplitude of a high frequency component becomes low in a range where the gradation conversion curve has a low slope by which an instructive information disappears even if the dynamic range is compressed with keeping the amplitude of the high frequency component. Furthermore, it has also a problem that an overshoot or an undershoot may occur in an edge portion.
Furthermore the above conventional vivifying method of highlighting a high frequency component does not have a concept of compressing the dynamic range while the strength of a high frequency component addition can be freely adjusted, and therefore it has a problem that an image having a wide range of a density distribution cannot be displayed on a single film sheet.
The present invention is provided to solve these problems and it is an object of the present invention to freely adjust a range of a density distribution of an image and an amplitude of a high frequency component.